Slow sand filters have been in operation for more than 150 years to provide water safe from bacteria, viruses, Giardia cysts and turbidity. Slow sand filters rely on a biologically active matrix that naturally develops on the top of the sand for high efficiency. Slow sand filters need to incorporate features that make them easy to install, and easy to maintain. Additionally slow sand filters should have features that protect the biological layer and carefully regulate flow for maximum efficiency. The advent of the use of plastic containers has resulted in a number of small packaged slow sand filters, including the current invention the SSF/x slow sand filter.
It was the inventors intention to design a package slow sand filter that required as little assembly by the end-user as possible, and further embodied a complete, efficient appearance after manufacture for turnkey installation and maintenance. In this regard, several problems with prior art packaged slow sand filters were noted by the inventor One problem is that most if not all package plant slow sand filters include hydraulic schemes that exit the filter at the bottom of the filter vessel and then have various piping systems that are assembled on or adjacent to the exterior of the filter (Manz U.S. Pat. No. 5,993,672, Pyper U.S. Pat. No. 5,032,261, Cluff U.S. Pat. No. 5,112,483, Simpson U.S. Pat. No. 5,264,129). This appeared to be a problem to the inventor because it requires additional unnecessary assembly steps prior to installation. This is a problem because there is the possibility of installing incorrect parts, loss of parts in shipment, and incorrect assembly of parts. If the exterior pipes are installed prior to shipping, it seemed probable that they would be vulnerable to breakage in transit, unpacking, or in installation. The inventor desired to produce a filter that was pre-assembled and self-contained as much as possible in order to avoid lost parts, use fewer parts, minimize possibility of breakage, and be as fool-proof as possible to install and use.
To address these issues, the inventor employed a novel piping scheme that collects water as normal at the bottom of the filter, but then proceeds through pipe work up the inside of the filter vessel exiting at a height of 36″ through a tank fitting in the vertical wall of the filter vessel. Several desirable features result from this novel piping scheme. This piping scheme prevents inappropriate piping from being installed and allows for the filter to be shipped in a more pre-assembled, self-contained condition further insuring proper installation. Placing the outlet of the filter above the filter sand level prevents the possibility of de-watering the delicate biological layer on top of the sand and thus killing it. Placing the outlet of the filter above the sand layer also provides less head pressure against the flow control device, making a larger orifice size available and thus making the flow control less susceptible to clogging. Since the piping system is pre-installed inside the filter vessel, it is not vulnerable to breakage in shipment, unpacking, or as maneuvered to install.
Secondly the inventor employed a device for flow control that attaches to the outside of the filter vessel tank fitting. This device incorporates a flow control being a neoprene washer with a precisely drilled hole that will only pass water at the rate prescribed for the filter. This eliminates guess work in installation and insures proper filter loading rate as calculated for the filter. This part of the apparatus is the only assembly on the outside of the filter. The inventor decided to place the flow control on the outside of the filter vessel so that it could be serviced without risking cross contamination possible if the flow control was located in proximity to the raw water within the filter vessel.
Thirdly, the inventor placed a valve emerging from the side of the filter vessel above the sand level to drain off the supernatant water above the sand during the wet harrowing process. Wet harrowing involves raking or stirring the top layer of sand and thereby releasing filtered material into the supernatent water above the sand. The Harrowing valve then discharges the now dirty supernatant water to waste. Wet harrowing does not require removing the top layer of sand during cleaning, or removing a geofilter cloth (Pyper U.S. Pat. No. 5,032,261). Resanding is not necessary because no sand is removed. The biological layer is not exposed to air, and is therefore back up to removal efficiency in minutes to hours instead of days.
Another problem with prior art plants is the lack of simple headloss measurement devices. It is important to measure headloss because short of failure of the system to continue processing water due to excessive buildup of filtered material, headloss measurement is the best way to determine when the filter needs to be cleaned. On prior art devices, headloss measurement devices are either not present (Pyper U.S. Pat. No. 5,032,261, Cluff U.S. Pat. No. 5,112,483, Simpson U.S. Pat. No. 5,264,129), or employ electrical sensors (Manz U.S. Pat. No. 5,993,672). The inventor felt that it was necessary to have a simple device to measure headloss so that the end user can tell from merely looking at the outside of the device when it needs cleaning. Further any headloss measurement device should not require power, because not all installation sites will have power available. Prior to the flow control is a tee fitting with a segment of clear PVC pipe extending vertically to the top of the filter and open at the top end. This structure acts as a piezometer, that is a device to measure headloss in the filter. As the filter is operated, the water level in the peizometer tube drops as headloss builds up in the filter. The piezometer needs to be on the outside of the filter vessel in order to be easily read. It is important to make sure that the piezometer is installed prior to the flow control device to measure properly. The inventor insures this by placing a ½″ male iron pipe threaded PVC fitting such that the flow control device can only be attached to the filter in the proper direction. Several surprising and unexpected results from these improvements include:
An unexpected result of locating the piezometer on the outlet piping assembly before the flow control is that it acts as a siphon break insuring that the filter cannot be inadvertently dewatered by siphon action working against the flow control device. An unexpected result of the internal piping scheme is that the piping is thereby freeze protected, unlike prior art systems.
An unexpected result is that the internal piping scheme is safer from vandalism than exposed piping on prior art systems.
An unexpected result is that in order for the fixed rate flow control to work effectively, it needs to be placed above the sand layer with no more than 12″ head pressure against it in a normal filtering mode. This supernatant level lower than traditional slow sand filters (36″) unexpectantly provides added oxygen levels to the biological layer of the filter.